This invention relates to receivers for decoding signals received via multiple propagation paths having different propagation delays and, more particularly, to delay and channel estimation for multi-carrier systems.
Radio receivers are often used for decoding fading signals with the aid of estimates of the instantaneous propagation channel phase and amplitude characteristics. An exemplary application for such a radio receiver is a cellular phone for decoding signals transmitted by base stations using code division multiple access (CDMA) protocols.
A radio receiver, such as for a CDMA system, receives digitally coded and modulated signals from a transmitter. These signals include known, preselect signal patterns at known time intervals. Using known signal patterns, also referred to as pilot sequences, or pilot channel as commonly used in the CDMA literature, the receiver forms successive estimates of the delay and amplitude or complex value of propagation path characteristics between the transmitter and the receiver. These include estimates for multiple paths in the case of multi-path propagation.
In a typical CDMA system a signal is transmitted on a single carrier. However, this can limit data rate of transmission. More recently, a multi-carrier (MC) system is proposed in which the signal is transmitted using three sub-carriers. This effectively triples the data rate. However, the MC system requires that filtering, correlation and de-modulation be performed for each of the three sub-carriers.
The common approach for MC CDMA system delay and channel estimation is to use three baseband filters. The signal for the first sub-carrier is extracted using the first filter. A first correlator correlates the filtered signal using the known sequence. From this, the delay and complex gain can be determined. This process is repeated for the second and third sub-carriers.
In general, the signal transmitted by the k'th carrier is given byuk(t)=[sk(t)+ok(t)]ejφk,  (1)where sk(t) is the known pilot signal of the k'th carrier, ok(t) is the sum of all other Walsh-orthogonal channels in the same carrier and ejφk is a constant phase term. Both sk(t) and ok(t) are baseband signals resulting from modulating data sequences on to streams of baseband pulse shape. The overall signal sent by the transmitter is                               g          ⁡                      (            t            )                          =                              ∑            k                    ⁢                                                    u                k                            ⁡                              (                t                )                                      ⁢                          ⅇ                              j                ⁢                                                                   ⁢                2                ⁢                                                                   ⁢                                  π                  ⁡                                      (                                                                  f                        0                                            +                                              f                        k                                                              )                                                  ⁢                t                                                                        (        2        )            where f0 is the center carrier frequency and fk is the frequency separation of the k'th carrier with respect to the center carrier frequency. The frequency spacing fk's are assumed wide enough so that there is no spectrum aliasing between sub-carriers.
After propagating through a multi-path channel, the RF received signal can be expressed as                                                                                           r                  ~                                ⁡                                  (                  t                  )                                            =                            ⁢                                                                    ∑                    i                                    ⁢                                                                                    c                        ~                                            i                                        ⁢                                          g                      ⁡                                              (                                                  t                          -                                                      τ                            i                                                                          )                                                                                            +                                                      n                    ~                                    ⁡                                      (                    t                    )                                                                                                                                          =                                ⁢                                                                            ∑                      i                                        ⁢                                                                                            c                          ~                                                i                                            ⁡                                              [                                                                              ∑                            k                                                    ⁢                                                                                                                    u                                k                                                            ⁡                                                              (                                                                  t                                  -                                                                      τ                                    i                                                                                                  )                                                                                      ⁢                                                          ⅇ                                                              j                                ⁢                                                                                                                                   ⁢                                2                                ⁢                                                                  π                                  ⁡                                                                      (                                                                                                                  f                                        0                                                                            +                                                                              f                                        k                                                                                                              )                                                                                                  ⁢                                                                  (                                                                      t                                    -                                                                          τ                                      i                                                                                                        )                                                                                                                                                                    ]                                                                              +                                                            n                      ~                                        ⁡                                          (                      t                      )                                                                                  ,                                                          (        3        )            where {tilde over (c)}1 is the complex channel gain of the multi-path, τ1 is its delay and ñ(t) is the Additive White Gaussian Noise (AWGN) with power spectral density N0.